Materials Science Forum Vols. 600-603

Abstract: SuperFETTM MOSFETs and silicon carbide (SiC) Schottky diodes are applied to continuous conduction mode active power factor correction pre-regulators. SuperFETTM MOSFETs can reduce power losses dramatically with their extremely low RDS(ON) and fast switching. The SiC Schottky diode has virtually zero reverse recovery current and high thermal conductivity, and is close to an ideal diode for a CCM PFC circuit. Due to these outstanding switching characteristics, frequency can be increased. In this paper, the SiC Schottky diode’s and SuperFETTM MOSFET’s performance have been verified in a CCM PFC boost converter. These products can reduce the total power losses and enhance the system efficiency.

Abstract: Due to research results have already been published.

Abstract: We succeeded in growing a GaN single crystal substrate with diameter of about two inches using the Na flux method. Our success is due to the development of a new apparatus for growing large GaN single crystals. The crystal grown in this study has a low dislocation density of 2.3×105 (cm-2), The secondary ion mass spectrometry (SIMS) technique demonstrates that the Na element is difficult to be taken in the crystal in both the + and – c directions, resulting in a Na concentration of 4.2 × 1014 (cm-3) in the crystal. Our success in growing a two-inch GaN substrate with a low impurity content and low dislocation density should pave the way for the Na flux method to become a practical application.

Abstract: The first commercially viable high voltage (>600V) gallium nitride (GaN) Schottky barrier devices are reported. Though GaN does not have any “micropipe” defects, which commonly exists in SiC material, defects like dislocations due to lattice mismatch hamper the material development of GaN high power devices. Improvements in the nitride epitaxial film growth have led to significant reduction of conductive dislocations. Conductive Atomic Force Microscope (CAFM) analysis of conductive dislocations shows only on the order of 103 cm-2 density of conductive dislocations, which are believed to be responsible for the undesired leakage current. GaN diodes compare to SiC or Si devices demonstrate a significant advantage in the thermal resistance. The insulating properties of Sapphire substrates allow fabrication of the devices in TO220 packages with insulating frame and thermal resistance better than 1.8°C/W compare to 3°C/W of SiC or Si devices with insulating frame. Performance of GaN, SiC and Si devices in the switch mode power supplies is compared.

Abstract: State-of-the-art technologies of GaN-based power switching transistors are reviewed, in which normally-off operation and heat spreading as technical issues. We demonstrate a new operation principle of GaN-based normally-off transistor called Gate Injection Transistor (GIT). The GIT utilizes hole-injection from p-AlGaN to AlGaN/GaN hetero-junction which increases electron density in the depleted channel resulting in dramatic increase of the drain current owing to conductivity modulation. The fabricated GIT on Si substrate exhibits the threshold voltage of +1.0V with high maximum drain current of 200mA/mm. The obtained on-state resistance (Ron·A) and off-state breakdown voltage (BVds) are 2.6mΩ·cm2 and 800V, respectively. These values are the best ones ever reported for GaN-based normally-off transistors. In addition, we propose the use of poly-AlN as surface passivation. The AlN has at least 200 times higher thermal conductivity than conventional SiN so that it can effectively reduce the channel temperature.

Abstract: GaN MOS capacitors were characterized to optimize the electric properties of SiO2/GaN interface. With optimized anneal conditions, an interface state density of 3.8×1010/cm2-eV was estimated at 0.19 eV near the conduction band and decreases deeper into the band gap. Enhancement-mode GaN MOSFETs were experimentally demonstrated on both p and n GaN epilayer with record high field-effect mobility of 167 cm2/V-s. Lateral RESURF-type GaN MOSFETs exhibit non-destructive high voltage (up to 940V) blocking capabilities. Other characterization including mobility orientation dependence, MOS-gated Hall mobility, current collapse and an NMOS inverter utilizing E/D mode GaN MOSFETs have also been experimentally demonstrated.

Abstract: To achieve AlN bulk growth, HTCVD chlorinated process is investigated. High growth rate and high crystalline quality are targeted for AlN films grown on (0001) α-Al2O3 and (0001) 4H or 6H SiC substrates between 1100 °C and 1750 °C. The precursors used are ammonia NH3 and aluminium chlorides AlClx species formed in situ by action of Cl2 on high purity Al wire. Both influences of temperature and carrier gas on microstructure, crystalline state and growth rate are presented. Growth rates higher than 190 μm.h-1 have been reached. Thermodynamic calculations were carried out to understand the chemistry of AlN deposition. AlN layers were characterized by SEM and θ/2θ X-Ray Diffraction. Their epitaxial relationships with substrates were deduced from pole figures obtained by X-Ray diffraction on a texture goniometer.

Abstract: Planar m-plane GaN was grown on (1100) m-plane 6H-SiC substrates using high-temperature AlN nucleation layers by metalorganic chemical vapor deposition. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) images showed striated features on the sample surface aligned along the GaN [1120] direction, which are perpendicular to those associated with (1120) a-plane GaN. The epitaxial relationship between the m-GaN and 6H-SiC was analyzed using high-resolution x-ray diffraction (XRD). In order to reduce the defect density, epitaxial lateral overgrowth (ELO) was carried out on an m-GaN template with mask stripes along the GaN [1120] direction, which makes the lateral growth fronts advance along the GaN c-axis. On-axis XRD rocking curves show that the full width at half maximum (FWHM) values for the ELO samples were reduced by nearly half when compared to those of the m-plane template without ELO. Clear atomic steps were observed in the wing regions by AFM. The absence of the striated features that are associated with the template could be indicative of the reduction of basal stacking faults in the ELO wings. Low-temperature photoluminescence (PL) spectra showed an excitonic emission at 3.47eV, a basal stacking fault (BSF)-related emission at 3.41 eV, and other defect-related emissions at 3.29 eV and 3.34 eV.

Abstract: Cubic SiC/Si (111) template is an interesting alternative for growing GaN on silicon. As compared with silicon, this substrate allows reducing the stress in GaN films due to both lower lattice and thermal expansion coefficient mismatch, and can provide better heat dissipation. In this work, we first developed the epitaxial growth of 3C-SiC films on 50mm Si(111) substrates using chemical vapor deposition. AlGaN/GaN high electron mobility transistors were grown by molecular beam epitaxy on these films. Both the structural quality and the electrical behavior of these structures show the feasibility of this approach.

Abstract: The growth process of semipolar GaN(10-12) on Si(001) offcut substrates with 3C-SiC buffer layers has been investigated. From XRD analysis, the difference in the crystal orientation between GaN(10-12) and 3C-SiC(001) has been found to be around 8˚ toward the [110] direction of the 3C-SiC templates. From TEM observations, a cubic-phase AlN seed layer is found to grow on 3C-SiC(001) templates, and the swift transition from the cubic phase to a hexagonal phase leads to the stable growth of hexagonal nitrides. Using 8˚-offcut Si substrates, it is possible to obtain a mirror-like surface of GaN(10-12) using an approximately 10-nm-thick AlN seed layer, which swiftly transitions from cubic AlN to hexagonal GaN.